This 50-year study revolutionizes our understanding of bald eagles

Jon Gerrard and Elston Dzus lead a now 50-year study of a thriving population of bald eagles in northern boreal Saskatchewan. Here they talk about the longevity of the research, key discoveries, the challenges of climbing trees and more.

For the last 50 years, ornithologist, pediatrician and Manitoba Liberal politician Jon Gerrard has been studying a thriving population of about 100 bald eagles on northern Saskatchewan’s Besnard Lake. Elston Dzus, an ecologist with Alberta-Pacific Forest Industries Inc., has been part of the ongoing research since 1984, when during his undergraduate at the University of Saskatchewan he surveyed all eagle and waterbird populations on Besnard and other nearby boreal lakes. Over the decades — and with the help of fellow biologists and naturalists, their spouses, children and now grandchildren — they've contributed much to what we know about these iconic birds.

Here, the researchers talk about this unprecedented study of the world’s longest-standing stable raptor population — a population which started in a place that wasn’t even thought to be eagle habitat, and at a time when many bird of prey populations had disappeared because of hunting and pesticides. For a feature story about these secluded nesting grounds, read the September/October issue of Canadian Geographic.

Map: Chris Brackley/Can Geo

On studying a single eagle population for 50 years

JON GERRARD: To my knowledge, this is the longest running study of a stable raptor population anywhere in the world. These eagles did not escape the effects of DDT [prior to it being banned across North America in the early 1970s], but populations elsewhere certainly bounced around more. Still others have gone up and down a lot based on the availability of prey. But the fish at Besnard are steady, so we have a stable group of eagles. Other experts have found the long-term population models (based on the data we’ve collected), to be quite remarkable.

ELSTON DZUS: Most such studies are typically short — you know, one to four years. But that really doesn’t give you information on trends, how things are doing over meaningful space and time. The value of long-term datasets increases the longer you monitor; it gives you a better understanding of population dynamics at a local scale and, arguably, at a larger spatial scale as well. In our eagle study data, you can actually see the post-DDT continental recovery of bald eagles through the 1970s and 80s. We get an increasing number of nests and an increasing number of breeding pairs through that time period, and then it starts to level off in the mid-1980s as the continental population is recovering and the lake is saturated with territories.

On some key observations

GERRARD: Bald eagles were hunted significantly in the U.S. up until 1940, when the Bald and Golden Eagle Protection Act was passed. After that, observations of wintering areas in Glacier National Park and along the Mississippi River showed a steady increase in eagle sightings, but then eagles across the continent had to contend with the effects of DDT [which led to reproductive failures in numerous bird-of-prey populations]. When we arrived on Besnard Lake in the late 1960s, what was probably happening was that the eagle population was just reaching — but wasn’t quite at — its saturation level of about 100 individuals. It hit this point around the mid-70s, when we started doing boat censuses.

One of the things we saw over time was an increasing proportion of adults over immatures, presumably because the younger eagles are being pushed out by the older, stronger birds. In the censuses that we regularly perform on the lake, we routinely see fewer immatures where there are successful nests. Adults are very territorial, so it follows that they’re keeping the juveniles out.

DZUS: What we saw, then, was something you wouldn’t have seen on a short-term dataset — a classic density-dependent response where the number of breeding adults increases and they’re forced to compete more for resources, including fish and space. So even though the total number of successful nests and occupied breeding areas went up and then stabilized, the number of young per nest eventually declined. Even though the total population has remained around 100 — the carrying capacity of the lake — there’s essentially a limit on the number of eagles that can thrive, breed, and the number of young that can be produced. We’ve had some banded adults that aren’t breeding until they’re six years old because they have to wait to find a territory.

A juvenile bald eagle reacts to being banded. (Photo: Elston Dzus)

On the importance of the census

GERRARD: In the mid-70s we realized we needed to know more than just how many nests and young there were — we wanted to know the total population and the different ages. So we developed a “boat census,” taking our boats along alternate eight-kilometre sections of shoreline. The rationale was that if you disturbed an eagle and it flew into the next section, you wouldn’t get into the problem of recounting birds. It also meant that we could survey half the lake and then double the number, which we later adjust slightly depending on the number of active nests counted and so on. That has given us a very reproducible number for bald eagles on the lake.

We use leg bands (and for a few years, patagial [wing] markers) to follow which birds are coming back, and found that we could actually use these resightings as a check on population estimates. It seemed that no one had ever done studies this way before, so we wrote it up and published the study. It provided a correlation that showed that our census numbers were in the right ballpark.

When we started the eagle censuses, we also began monitoring the area’s ducks, gulls, loons, grebes and other waterbirds. Every few years since then we’ve done full lake censuses just to see what was happening long-term. That’s given us data on significant changes in these other populations: for example, ring-billed gulls went up, then came down; Bonaparte’s gulls, which were not plentiful to start with, are now quite plentiful.

DZUS: This comes back to what my Master’s thesis looked at: what are the driving factors influencing higher bald eagle density on different lakes? In my study of Besnard and nearby Nemeiben Lake I compared human disturbance and the amount of suitable nesting habitat between the two lakes, which are only 40 kilometres apart. There wasn’t much difference on those fronts, but what made Besnard so much more productive was its food resources. Two-thirds of it sits on the Precambrian Shield and the bottom third is on the Boreal plain. Nemeiben, for the most part, is on the shield. That means nutrient-rich water flows into the Mercer River from the plains and enters the west side of Besnard Lake. This nutrient loading and higher water quality leads to more invertebrates, which leads to more fish, which leads to more eagles. That kind of data can be applied to comparisons between lakes that have been influenced by human industry, climate change and other factors.

On discovering bald eagle sex-regulation

GERRARD: Gary Bortolotti, an avian biologist who started coming to Besnard Lake in the mid-70s, based his PhD thesis on the problem of distinguishing between male and female young in the nest. They’re very similar in appearance, and although we knew females were bigger than males, we didn’t have a way of clearly differentiating between them. Bortolotti measured a lot of museum specimens with known sexes as well as the young on Besnard lake, and he found a clear separation.

Interestingly enough, what proved to be most important was the length of the foot pad on the talon and the depth of the beak. Combining those two measurements provided a very accurate separation of males and females. With that information, Bortolotti started climbing trees to study nests that had eggs and very small young, and followed them all along. Now, you would predict there would be equal numbers of males and females in both the first and second waves of hatchlings, but in the years he was performing his study, which were very productive in terms of chicks, it turned out that there was an excess of females hatching first and males second. When Elston later looked at less productive Nemeiben lake, he found more males first and females second. Those ratios have held true for good years and bad years on Besnard.

Why is that? One presumes that where there’s more prey, there’s an advantage to having females hatch first, because they’re larger and require more food as they’re being raised. In poorer years or habitats, there’s an advantage to having males first because they require less food and less energy. What we found was a fascinating apparent ability of bald eagles to regulate the sex of their first young in particular, depending on the conditions of the lake.

On nesting habits

DZUS: There are approximately 50 nests on the lake. Some breeding areas have two, occasionally three nests, and the birds will switch year-to-year, but most breeding areas only have one nest. These will be reused repeatedly for anywhere from one to eight years or even more, sometimes until the tree falls or the eagles just decide to switch. We’re not always sure why they do this. There are theories that it’s parasite loading, but no one has good, definitive information on that.

Two of the birds that we banded as hatchlings on Besnard Lake started returning as breeding adults when they were six years old. One of them continued to return year after year to the same breeding area — from when it was six until it was 18 years old. That helps generate a bit of information on longevity. Unfortunately, we aren’t certain how many mates it went through during that time period, because the other bird in the pair wasn’t banded.

On nesting sites and their dangers

DZUS: Besnard is surrounded by mixed-wood boreal forest. Anywhere you’ve got peatland wetlands, you’re going to have a lot of black spruce, but it’s the white spruce here that the eagles prefer, which are larger and sturdier, better for supporting huge nests.

The majority of nests built on the lake’s islands are in white spruces. Interestingly though, if a nest is on the mainland, it’s more likely to be in a trembling aspen, which are also common in the area. I would say that goes broadly for the boreal forest. This is because bald eagles choose the largest, oldest trees in their territories, and aspens grow faster than spruce following wildfires on the mainland. The probability of a fire hitting an island is much lower, of course, so white spruces are more likely to get big enough to hold eagle nests.

In 2015, we had massive fires all across the northern part of Besnard Lake that affected about eight breeding areas. Two more nests were in burn areas in 2017. A massive wind storm on July 16, 2017, brought down two of those fire-affected trees, along with the nests and young. When my wife, Connie, and I went to check on the sites, we found the first chick on the ground, still alive. When this kind of thing happens, the adults will still continue to feed the young until they fledge. The other nest was intact, leaning against a cliff, and the chick was healthy and happy and standing on the ledge. It will be fine.

Katherine Conkin and Elston Dzus take a moment to pose with a newly banded immature bald eagle before it's sent back up the tree to Jon Gerrard, who will return it to its nest. (Photo: Elston Dzus)

On climbing trees, eagle attacks and new technology

GERRARD: I like to tell people that I’m one of the few people who spends their summer holidays climbing trees. I don’t climb as many as I used to, but I still get up there to count and band offspring. Until the last few years I spent more time climbing, including up nearby trees to get a better view of what was in nests, but now that we’re using a drone that’s not as often necessary.

Early on, the eagles were more likely to fly up and get excited if we went near their nests, but they’re generally quite tolerant of our presence. Only once in our 50 years have I had an eagle dive at me while I was at the top of a nest. They tend, instead, to perch nearby or fly overhead without bothering us, and on nests where there’s a canopy, such as on an aspen, it’s much harder for the eagles to dive in. The drone takes much less time than climbing, depending on the circumstances, so now we’re causing even fewer disturbances.

DZUS: There are certain raptor species that are quite aggressive and will actually strike at people and drones, but as we predicted, the birds on Besnard have not shown a territorial or defensive response. We haven’t even needed protective equipment when dealing with these bald eagles — I’ve only had one come remotely close to me on a climb since 1984. They’re apex predators, at the top of the food chain, and there are few things they feel the need to defend their nests against. Compare that to the fact that I have seven holes in my back from a great horned owl that struck me. Raptor banders who are involved with owls, falcons or many of the hawk species are often and repeatedly attacked, so they wear leather jackets, helmets and leather napes on their necks.

We’ve been employing drone technology only for the last three years, and it’s making a big difference to our data collection, saving us on climbing and time. You see, it used to be that if I travelled 30 kilometres by boat to visit a nest and I either saw no young or only one young, I had to come back later to perform another check, to be sure. (If I saw two young, I could be confident that’s all there was.) Now, if I see no chick, one chick or there’s an adult present, I can send the drone up to very quickly determine the accurate count.